1. Field of the Invention
This invention relates to a post-objective type optical deflection apparatus, and particularly to an objective type optical deflection apparatus using a polygonal mirror.
2. Description of the Prior Art
Among optical scanning apparatuses for the optical system used for a laser printer, a laser display and the like, a post-objective type deflection apparatus in which an optical deflector is arranged after the beam has been formed into a converging luminous flux by means of a convergent lens is designed so that the beam is subjected to converging action prior to deflection, and therefore a problem encountered in the convergent lens is only the axial image formation. The post-objective type deflection apparatus has an advantage in that the lens construction is simple. On the other hand, converging points are generally on a curved surface, and the apparatus is not suitable for plane-scanning required by the laser printer, the laser display and the like. In addition, even if the curvature of field is corrected by some kind of means, the image extends as it moves toward the scanning end, and the so-called f.theta. characteristic, which requires the proportional relation between the deflection angle and the scanning line, deteriorates. (It excessively increases toward a plus.) In the pre-objective type in which the convergent lens is arranged after deflection, the convergent lens has to be designed in the form of a wide angle enough to cover the deflection angle, which is liable to have the lens structure complicated. However, this concept can be used to facilitate correction of the curvature of field and the f.theta. characteristic. This is the reason why the pre-objective type is now principally used.